1. Field
The disclosure relates generally to a bandgap reference circuit and, more particularly, to a bandgap reference circuit device for a low voltage power supply thereof.
2. Description of the Related Art
Bandgap reference circuits are a type of voltage reference circuit used in conjunction with semiconductor devices, integrated circuits (IC), and other applications. The requirement for a stable reference voltage is universal in electronic design. Using a two transistor circuit, collector current sensing can provide minimization of errors due to the base current. In a bandgap voltage reference circuit, the voltage difference between two p-n junctions (e.g. diodes, or bipolar transistors), operated at different current densities, can be used to generate a proportional to absolute temperature (PTAT) current in a first resistor. This current can then be used to generate a voltage in a second resistor. This voltage, in turn, is added to the voltage of one of the junctions. The voltage across a diode operated at a constant current, or herewith a PTAT current, is complementary to absolute temperature (CTAT). If the ratio between the first and second resistor is chosen properly, the first order effects of the temperature dependency of the diode and the PTAT current will cancel out. In this fashion, a circuit can be independent of temperature variation, and provide a constant voltage reference.
Circuits of this nature that are temperature insensitive are referred to as bandgap voltage reference circuits. The resulting voltage is about 1.2-1.3V, depending on the particular technology and circuit design, and is close to the theoretical silicon bandgap voltage of 1.22 eV at 0 degrees Kelvin. The remaining voltage change over the operating temperature of typical integrated circuits is on the order of a few millivolts. Because the output voltage is by definition fixed around 1.25V for typical bandgap reference circuits, the minimum operating voltage is about 1.4V.
A circuit implementation that has this characteristic is called a Brokaw bandgap reference circuit. The Brokaw bandgap reference circuit is a voltage reference circuit that is widely used in integrated circuit technology to establish a voltage reference. The Brokaw bandgap reference circuit has an output voltage of approximately 1.25V, and has very little temperature dependence. Like all temperature-independent bandgap references, the circuit maintains an internal voltage source that has a positive temperature coefficient (PTC) and another internal voltage source that has a negative temperature coefficient (NTC). By summing the two together, the temperature dependence can be canceled. Additionally, either of the two internal sources can be used as a temperature sensor. In the Brokaw bandgap reference, the circuit uses negative feedback to force a constant current through two bipolar transistors with different emitter areas. The transistor with the larger emitter area requires a smaller base-emitter voltage, VBE, (e.g. or Vbe) for the same current. The base-emitter voltage for each transistor has a negative temperature coefficient (i.e., it decreases with temperature). The difference between the two base-emitter voltages has a positive temperature coefficient (i.e., it increases with temperature). With proper component choices, the two opposing temperature coefficients will cancel each other exactly and the output will have no temperature dependence.
FIG. 1 shows the circuit schematic for the Brokaw bandgap circuit. The circuit is powered using a VDD power supply 10, and ground power supply 20. The output of the circuit is VREF 30. The circuit contains a first npn bipolar transistor, NPN1 70, and a second npn bipolar transistor, NPN2, 80. The circuit network contains a first resistor R1 90, and a second resistor R2 95 to provide a feedback network. A first p-channel MOSFET, PMOS1, 40 and second p-channel MOSFET, PMOS2 50, serve as current sources for the NPN1 70 and NPN2 80 transistors, respectively. For the output voltage reference, a third p-channel MOSFET, PMOS3 60 and resistor R3 100 are provided. The base-emitter voltage of NPN 1 70 is referred to as Vbe1. The base-emitter voltage of NPN2 80 is referred to as Vbe2. The voltage drop across first resistor R1 90 is kT/q ln {J1/J2} where J1 and J2 is the current density flowing through NPN1 70, and NPN2 80, respectively. The voltage drop across second resistor R2 95 is equal to 2 (R2/R1) {kT/q} ln {J1/J2}.
The first and second npn bipolar transistors, NPN1 70 and NPN2 80, are of different physical size. When the voltage at their common base is small, the voltage drop, the voltage drop across the resistor R1 90 is small, the larger area of NPN2 causes it conduct more of the total current available through R2 95. The resulting imbalance in collector voltages drives the op amp so as to raise the base voltage. Alternatively, if the base voltage is high, a large current is forced through R2 95 leading to a voltage drop across R1 90 will limit the current flow through NPN2 80. Between the two extreme imbalance of the collectors, is a base voltage at which the collector currents match. The npn layout of NPN1 70 and NPN2 80 can have different “emitter finger” numbers to establish different physical bipolar transistor sizes. This can be designated by 1:M, indicating the ratio of emitter fingers of 1 in NPN1 70 and multiplicity of M in NPN2 80. The two transistors establish a differential voltage in the base-emitter voltage, referred to as a so-called “delta Vbe voltage” (Δ Vbe),ΔVbe={kT/q} ln [J1/J2].Hence, the differential voltage, ΔVbe, is proportional to the current flowing through the NPN1 70 and NPN2 80. The collector current flowing through collector of NPN1 70 is equal to the current flowing through PMOS1 40. The collector current flowing through NPN2 80 is equal to the current flowing through PMOS2 50. Therefore, PMOS1 40 and PMOS2 50 current ratio is proportional to “delta” Vbe voltage, which is proportional to absolute temperature (PTAT). Assuming the resistor ratio and current density ratio are invariant, the voltage varies directly with absolute temperature T.
In this network, the base of NPN1 70 and NPN2 80 are electrically connected, providing a common base voltage condition. The base voltage of NPN1 70 and NPN2 80 base (which are connected) can be calculated according to the following
      R    ⁢                  ⁢          2      ·                        Δ          ⁢                                          ⁢          Vbe                          R          ⁢                                          ⁢          1                      +  VbeThe Delta Vbe, Δ Vbe (also denoted as D Vbe), and Vbe can cancel out the temperature coefficient, because Δ Vbe has a positive temperature coefficient (PTC) and Vbe has a negative temperature coefficient (NTC) and creates the temperature independent, so-called bandgap voltage. The utilization of the transistor base-emitter diode temperature compensated to the bandgap voltage of silicon is a technique to establish the bandgap voltage. This is usually becomes approximately ˜1.2V.
However, in this Brokaw bandgap circuit, the common base voltage, VB, of NPN1 70 and NPN2 80 is approximately ˜1.2V. Therefore, the collector voltage of NPN1 70 should be higher than the value of ˜1.2V. The p-channel MOSFET, PMOS1 source and gate voltage needs to be more than a threshold voltage of a PMOS transistor, which usually more than 0.4V˜0.7V; in addition, some additional voltage is desirable to provide good matching PMOS1 40 and PMOS2 50. Therefore, in this case, the supply voltage must be more than 1.6V. Additionally, a supply voltage of more than 2V is desirable in this circuit. It is difficult to make this circuit operable in ideally, a 1.5V power supply voltage range.
With technology scaling, according to constant electric field scaling theory, the power supply voltage, VDD, continues to decrease to maintain dielectric reliability. In current and future semiconductor process technology, having minimum dimensions of for example, 0.18 μm, and 0.13 μm, the native power supply voltage (or internal power supply voltage) is 1.5V internal supply voltage for digital circuits, and other sensitive analog circuitry. In the range of 1.5 V power supply voltage, the Brokaw bandgap reference does not provide desirable operability, or ideal operating characteristics. For technologies whose minimum dimension is below 0.13 μm, the issue is also a concern.
In bandgap voltage references, usage of Brokaw style bandgap regulators have been discussed. As discussed in U.S. Pat. No. 7,208,930 to Tran et al, a Brokaw style bandgap voltage regulator is described comprising a that comprise of bipolar current mirror, resistor divider networks and a bipolar output device.
In bandgap voltage references, bandgap circuits on the order of 1V have been discussed. As discussed in U.S. Pat. No. 7,199,646 to Zupcau et al., a bandgap circuit topology allows for function at low power supply voltages, on the order of 1V. The circuit has a replication circuit, and an output stage containing a current mirror.
In bandgap voltage references, bipolar assembly and mirror assembly have been shown. As discussed in published U.S. Pat. No. 6,946,825 to Tesi et al. shows a circuit with a current mirror assembly of cascode type.
In bandgap voltage references, alternative embodiments have been discussed. As discussed in U.S. Pat. No. 5,982,201 to Brokaw et al, a bipolar current mirror network, a resistor divider network, and an output transistor allow for operation with supply voltages of less than two junction voltage drops.
In these prior art embodiments, the solution to improve the operability of a low voltage bandgap circuit utilized various alternative solutions.
It is desirable to provide a solution to address the disadvantages of the low voltage operation of a bandgap reference circuit.